Prior art devices of this general character have been proposed, some of which become completely disengaged and cease their driving function, as well as signalling the overload condition. An example of such a prior art device is shown in the U.S. Pat. No. 2,144,769 issued Jan. 24, 1939 and entitled "Overload Release Structure". That device is furthermore extremely complicated and operates with a cushioning or gentle action when tripping on overload is encountered, which results in an intended no shock load condition on the machine it is driving. The resulting structure however results in a time lag in giving the overload signal which can be detrimental to the machine. That device may also result in "hunting" between the parts before the actual device is tripped and the overload signal transmitted.
Another example of a prior art device of this general character is shown in my U.S. Pat. No. 3,893,553 issued July 8, 1975 and entitled "Overload Release Clutch". That clutch is uni-directional in function and the switch plate is spring-loaded, but those springs do not function to set the maximum torque condition of the drive unit.